Pass a pointer to your callback function, which should match the prototype
shown above.

We encourage users to use the newer CURLOPT_XFERINFOFUNCTION(3) instead,
if you can.

This function gets called by libcurl instead of its internal equivalent with a
frequent interval. While data is being transferred it will be called very
frequently, and during slow periods like when nothing is being transferred it
can slow down to about one call per second.

clientp is the pointer set with CURLOPT_PROGRESSDATA(3), it is not
used by libcurl but is only passed along from the application to the callback.

The callback gets told how much data libcurl will transfer and has
transferred, in number of bytes. dltotal is the total number of bytes
libcurl expects to download in this transfer. dlnow is the number of
bytes downloaded so far. ultotal is the total number of bytes libcurl
expects to upload in this transfer. ulnow is the number of bytes
uploaded so far.

Unknown/unused argument values passed to the callback will be set to zero
(like if you only download data, the upload size will remain 0). Many times
the callback will be called one or more times first, before it knows the data
sizes so a program must be made to handle that.

Returning a non-zero value from this callback will cause libcurl to abort the
transfer and return CURLE_ABORTED_BY_CALLBACK.

If you transfer data with the multi interface, this function will not be
called during periods of idleness unless you call the appropriate libcurl
function that performs transfers.

CURLOPT_NOPROGRESS(3) must be set to 0 to make this function actually
get called.